1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having improved characteristics, by means of gettering technique.
2. Description of the Related Art
Contaminants, such as the unwanted impurities (heavy metals, among others) unavoidably introduced into a semiconductor substrate in the manufacture of a semiconductor device, form centers which trap and release free electrons (or holes), causing a leakage current flowing at a pn junction. The leakage current degrades electrical characteristics of the semiconductor device. In a MOS semiconductor element, for example, such a leakage current reduces the mutual conductance or the like, inevitably decreasing the yield of the semiconductor device. The heavy metals, even if existing in a very small amount, may greatly reduce the yield or characteristics of the semiconductor device, particularly where the device has a more limited size and yet comprises a greater number of elements, as has been being done recently.
To prevent such contamination, gettering is performed to absorb heavy metals. More specifically, the back of a semiconductor substrate is mechanically damaged, or impurities are introduced into the back of the substrate in high concentration, in order to absorb the heavy metals. In order to achieve this back-surface treatment successfully, an additional step must be carried out in order to prevent contamination from the front surface of the substrate, or the substrate must be heated at high temperature for a long time. The prolonged heat treatment is, in most cases, detrimental to the manufacture of a microstructure device. Further, the back-surface treatment is one of the initial steps of manufacturing the semiconductor device, and its effect gradually lessens as the succeeding heat treatments or thermal steps are performed.
Soft error, which is caused by excess carriers generated upon application of radiation such as .alpha.-rays, is a problem prominent to semiconductor memories. It is regarded as a great problem, in particular, to 64 KB dynamic RAMs and dynamic RAMs having greater memory capacities, and also as an equally great problem to static RAMs having high-resistance load cells. Soft error is discussed in "LSI HANDBOOK" edited by The Institute of Electronics and Communication Engineers of Japan, pages 677-678, Nov. 30, 1984, published by OHM Inc., Japan.
Various methods are taken to prevent soft error. Among these methods are: to reduce the amount of the .alpha.-ray sources, such as U and Th, contained in the package material (i.e., resin composition); to design the circuitry to increase the critical charge at the expense of the number of bit lines and the sensitivity of the sense amplifier; and to coat the LSI chip with a resin layer thick enough to prevent the .alpha.-ray penetration into the chip. The .alpha.-ray penetration from within and without the package can be reduced to a minimum by, for example, forming an .alpha.-ray shield made of polyimide resin or the like on the LSI chip which has been passivated. However, radioactive elements such as U and Th may be contained also in the Al layers or the high-melting metal layers which are formed on the LSI chip, and the shield of polyimide resin cannot block the radiation from these metal layers. Further, coating of polyimide resin may raise another technical difficulty in resin-packaging the LSI chip. Hence, it is desirable that the LSI chip itself be made resistant to soft error.
Also in a static RAM, the data charge accumulated in each memory node is so small that the data may be destroyed when the memory node is applied with an electrical charge generated when the .alpha.-rays penetrate into the static RAM. When .alpha.-rays hit the n.sup.+ layer located in the memory node, in particular, funneling takes place, generating carriers which flow back along the loci of the .alpha.-rays and absorbed into the memory node efficiently, inevitably causing a soft error.
As has been pointed out, in order to prevent contamination by heavy metals, it has hitherto been necessary to perform an additional step of preventing contamination from the front surface of the substrate in the backside gettering technique. In addition, since the gettering site is formed in one of the initial steps of manufacturing the semiconductor device, the effect of the gettering gradually lessens as the succeeding thermal steps are performed. Further, to achieve the gettering successfully, the substrate must be heated at high temperature for a long time, but the prolonged heat treatment is, in most cases, detrimental to the manufacture of a microstructure device.
Further, in semiconductor memory devices, soft error occurs due to excess carriers generated by .alpha.-rays.